A patient with hypertiglyceridemia with elevated apo C-III that inactivates lipoprotein lipase and interferes with apo-E receptor-mediated uptake of remnant particles by the liver has been identified, providing a unique opportunity to study the nature of the biochemical defect and the contribution of apo C-III to the pathophysiology of hyperlipidemia, to assess the effect of lipid-lowering diets on the properties of apo C-III and to study the genetics of this disorder.